From everything I've heard, the tiny hippos have a pretty poor survival rate to begin with.
In a sufficiently large tank, multiple tangs can get along fine. I had intended to introduce all of my tangs at once, but logistically, it's difficult to do when you QT your fish.
I stated with an achilles tang. A few months later I purchased a yellow, purple, and PBT. The PBT didn't make it through QT, but the other two did. The achilles had almost zero interest in the yellow, but initially chased the purple whenever he was out in the open. Over time, the harassment lessened, but never disappeared completely.
Since then, I bought a second PBT, put him through QT, and he has recently joined the group. The achilles has focused all of his attention on the PBT, and is completely cool with both the yellow and purple tangs. The PBT swims the entire tank, but is occasionally chased. Over time, this seems to be lessening, but hasn't gone away completely. Fortunately, the achilles and PBT are on equal footing in terms of size/speed.
I often joke that I need to find a rubber or plastic tang (Maybe a Dori toy from Nemo?) to throw in the tank for a few days to break up the squabbles. It's almost as if they set aside their differences and unite against the new guy when another fish is added.
The important point to take away is that the oldest fish will attempt to dominate the new ones. If the tank is big enough to allow them to get away, and provides safe places in the rocks, this may be tolerable. I'm sure it's stressful on the fish. If it weren't for QT and preventative ich treatment, I'm not sure I would be as successful.
Thanks!
